1. Field of the Invention
The invention relates to a method and device for determining a fill level of liquid in a container, in particular a level of fuel in a fuel tank of a motor vehicle.
2. Description of the Related Art
Lever transmitters with a float are typically used to determine a fill level of a liquid in a container. The float is connected to a pivotally mounted lever so that the float follows the liquid fill level in the container and pivots the lever which is connected to a potentiometer. The output of the potentiometer is a function of the fill level. The potentiometer is connected to an evaluation unit which converts electric signals into a fill level which corresponds to the volume of liquid present in the container. A disadvantage of this device is that the pivotal lever requires very large dimension of movement space inside the container making it difficult to arrange the lever in fuel tanks of modern motor vehicles which comprise complicated shapes to achieve compactness.
It is an object of the present invention to create a method for determining the fill level of liquid in a container which can be applied virtually independently of the geometry of the container to determine the fill level. Furthermore, it is also an object of the present invention to provide a device for determining the fill level of liquid in a container of virtually any shape.
The object of the present invention is met by a method including the step of completely filling or emptying a measuring chamber at a known pressure difference, the measuring chamber extending above the maximum level of the fuel tank to be measured and connected to the fuel tank. The time required for the filling or emptying of the measuring chamber is then determined and converted into the fill level in the container.
This method determines the fill level of liquid in the container via the rate at which the measuring chamber is emptied or filled. Since the measuring chamber is connected to the fuel tank, a fill level in the measuring chamber at the starting point of the method according to the invention, i.e., before the step of measuring or emptying the measuring chamber, corresponds to the fill level in the fuel tank. The time interval required for completely filling or emptying the measuring chamber is therefore a measure of the fill level in the container. The geometry of the measuring chamber and of the fuel tank exerts virtually no influence during the determination of the fill level using the method according to the invention. Accordingly, the measuring chamber does not have to be arranged vertically and may therefore be arranged at a slant or be bent.
The determination of the fill level in the container requires a particularly low computational outlay when the time determined for filling the measuring chamber is compared with a previously determined volume/level characteristic. The measuring chamber may therefore additionally have varying cross sections from the top end to the bottom end.
The object of the present invention is also met by a measuring chamber connected to the container which extends above the maximum level to be measured. The measuring chamber has a connection to a driven liquid delivery means and is designed to determine the time interval required for filling or emptying the measuring chamber. An evaluation unit is designed to calculate the fill level in the container from the time interval to determined.
This configuration according to the present invention requires no lever transmitter of projecting shape. Since the measuring chamber is connected to the container, the liquid respectively has the same fill level in the measuring chamber and in the container at the time before the level is to be determined. The time interval which is required to empty or fill the measuring chamber at a known delivery rate is a measure of the fill level in the fuel tank. The geometry of the measuring chamber and of the fuel tank exerts virtually no influence during determination of the fill level using the device according to the present invention. Therefore, the measuring chamber may, for example, be arranged at a slant or be bent. The measuring device may also subsequently use the fill level to calculate the volume of liquid in the container.
In accordance with an embodiment of the present invention, the measuring chamber is designed with a particularly simple shape such as, for example, a tubular shape.
The fact that the measuring chamber has been emptied or filled to a maximum may be determined, for example, via the power consumption of an electric motor driving the liquid delivery means. In accordance with another embodiment of the present invention, the device may be operated using virtually any desired liquid delivery means when a switch is arranged in the measuring chamber which is actuated via a float.
In accordance with another embodiment of the present invention, the fill level of liquid may be determined particularly quickly when the measuring chamber is connected to the container via an opening in the upper region of the measuring chamber and another opening in the lower region of the measuring chamber and when the lower opening is closable during filling or emptying of the measuring chamber.
The fill level in the container may be determined with particularly high accuracy when the measuring chamber is connected to the container via a plurality of openings arranged at different levels. It is possible thereby, for example, for liquid to escape through the openings into the container during filling of the measuring chamber. This lengthens the time interval required to fill the measuring chamber, with the result that measuring errors in the determination of the start and end of the measurement exert a small influence on the fill level determined.
In another embodiment of the present invention, the accuracy of the fill level determined is increased by using a liquid delivery means having a pressure reservoir. The device according to the invention therefore needs no delivery pump of its own.
The device according to the invention may be operatively arranged for driving the liquid delivery means for emptying the measuring chamber at prescribed time intervals, thereby determining the time required for emptying the measuring chamber at the prescribed time intervals.
In accordance with yet another embodiment of the present invention, the evaluation unit comprises a memory including volume/level characteristics determined for filling or emptying, and/or for fill level values last determined. As a result, the fill level in the container may be determined with very high accuracy even when the measuring chamber has varying cross sections. A section of the measuring chamber which has a large cross section may be used to determine the fill level in this region with particular exactitude. A low residual A quantity of fuel may be determined particularly precisely in the case of the device provided for measuring the fill level in the fuel tank when the measuring chamber has a large cross section in the area proximate the bottom of the container.
In accordance with a further embodiment of the present invention, a temperature sensor may be arranged in the container and connected to the evaluation unit so that the influence of temperature on the viscosity of the liquid in the container may easily be accounted for. This embodiment is especially useful for measuring the fill level of fuel in motor vehicles, since the viscosity of fuel has a strong dependence on temperature fluctuations.
Pressure changes in the liquid filling the measuring chamber may be produced during operation of the liquid delivery means which may, for example, comprise a delivery unit including a separate pump or pressure reservoir. The pressure changes produce falsification of the values determined. This falsification may be countered by arranging a pressure sensor at the outlet of the liquid delivery means or in the line leading to the measuring chamber, and connecting it to the evaluation unit to account for pressure changes.
The device according to the invention is of particularly cost effective design when an electrically operable delivery unit inserted in the fuel tank of the motor vehicle is designed as liquid delivery means.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.